How did John Davidson Rockefeller get rich?

in the oil industry

by selling supplies to the army

by mining diamonds and gold

by building railroads

John Davison Rock­e­feller was the founder of the Standard Oil Company, which dom­i­nat­ed the oil industry in the U.S. As kerosene and gasoline grew in im­por­tance, Rock­e­feller's wealth soared, and he became the world's richest man and first American worth more than a billion dollars.